Local area networks are commonly used to allow multiple computing devices to communicate over distances. Examples of local area networks are networks which comply with the ANSI/IEEE 802.3 standard. Such networks use transceivers to interface between the computing device, such as a host computer, and the local area network medium, such as a coaxial cable.
Transceivers commonly communicate with the host computer via three differential signals: data-in, date-out, and collision. Messages that the transceiver receives from the network medium are supplied to the host computer via data-in, and data-out is asserted by the host computer to transmit messages. The collision signal is asserted by the transceiver when a collision between multiple messages is detected on the network medium. However, the collision signal may also be asserted as an acknowledgement that the transceiver has received a data-out signal from the host computer and has transmitted the message on the network medium. This acknowledgment is known as a heartbeat. Timing specifications insure that collision signals occurring within 1.0 .mu.sec of the end of a transmission are heartbeats, while later collision signals result from the detection of data collisions on the network medium.
In some cases, the host computer is remote from the transceiver, requiring the use of one or more repeaters to boost signals sent between the host computer and the transceiver. Local area network specifications, such as ANSI/IEEE 802.3, however, prohibit the use of heartbeat signals when repeaters are employed. Thus, the transceiver must provide a method of enabling or disabling the heartbeat transmissions to the host computer depending on the presence of repeaters.
The enabling and disabling of the heartbeat transmissions is typically done by controlling the voltage at an input of the transceiver, known as the heartbeat enable input. The transmission of heartbeats is typically enabled when this input is held above a voltage threshold, and disabled when the input is held below a threshold. In addition, a light-emitting diode is sometimes turned on and off in conjunction with the heartbeat enable signal so that a user may easily observe the state of the transceiver. The light-emitting diode is typically lit when the heartbeat is enabled, and unlit when the heartbeat is disabled.
The control of the heartbeat enable signal, in conjunction with the lighting of the light-emitting diode, is typically performed by a circuit containing a single-pole double-throw switch. In its first position, the switch completes an electrical path between differential supply voltages which includes a series light-emitting diode. In this position, the switch also allows the heartbeat enable input on the transceiver to be pulled up to a voltage above a voltage threshold. Thus, in its first position, the single-pole double-throw switch enables the heartbeat and lights the light-emitting diode. In the second position, the switch typically clamps the heartbeat enable input below a voltage threshold and breaks the series circuit containing the light-emitting diode. This turns the light-emitting diode off and disables the transmission of heartbeat signals by the transceiver. While effective, the use of a single-pole double-throw switch exceeds the cost of simpler switches.
It is desirable, therefore, to have a simple and inexpensive circuit which is suitable for controlling the heartbeat enable signal supplied to a transceiver. It is also desirable that the circuit simultaneously activate a light-emitting diode or other signaling device when the heartbeat signal is enabled. Further, it is desirable that the circuit operate using a simple and inexpensive single-pole single-throw switch.